I have done some permanent slides using microscope slide and cover slip and nail varnish. If the object is not too big (most cases on mesozooplankton it is too large) then it should work nicely. Just but a little drop of fixated (4% formaldehyde or lygol or spirit+clycerin) plankton species on microscope slide and cover it with cover slip and then cover the edges of covr slip with nail varnish (transparent is the best and some companies make better varnish than others). Let the varnish dry and your slide would be ready to preserve!